1. Field of the Invention
This invention refers generally to bipolar transistors and more specifically to a thermally stable cascode heterojunction bipolar transistor fabricated on GaAs or other III-V compound semiconductors being used as active devices in microwave and high speed digital circuits.
2. Description of the Related Art
High power microwave heterojunction bipolar transistors (HBT) exhibit thermal instability, or thermal runaway, related to failures when operated under large direct current (dc) or radio frequency (RF) drive conditions. The basic cause of this instability is the negative temperature coefficient of the emitter-base turn-on voltage and the strong electrothermal feedback in devices with moderate to high thermal resistances. When a multi-finger HBT is biased from a single base voltage, as shown in FIG. 1, the electrothermal feedback can cause one of the emitter fingers to conduct most of the available current to the whole device and therefore create a xe2x80x9chot spot.xe2x80x9d
Thermal instability in HBTs can be reduced by the use of ballast resistors in series with each emitter or base finger, or by thermal-shunt techniques. See, G, B. Gao et al., Emitter Ballasting Resistor Design for, and Current Handling Capability of AlGaAs/GaAs Power Heterolunction Bipolar Transistors, IEEE Trans. Electron Dev., Vol. 38, pp. 185-196, 1991; W. Liu et el., The Use of Base Ballasting to Prevent the Collapse of Current Gain in AlGaAs/GaAs Heterolunction Bipolar Transistors, IEEE Trans. Electron. Dev., Vol. 43, pp. 245-251. 1996; B. Bayraktaroglu et al., Very High Power Density CW Operation of AlGaAs/GaAs Microwave Heterojunction Bipolar Transistors, IEEE Electron. Dev., Vol. 14, pp. 493-495, 1993. The stability achieved with ballast resistors usually come at the expense of reduced microwave performance, such as microwave gain and power-added efficiency (PAE). The reduction in power gain due to ballast resistors is especially undesirable at X-band and higher frequencies, where the power gain is already limited. PAE above 50% is more difficult to achieve at these frequencies, since the higher efficiency amplifier modes require high external device transconductance. Further, emitter ballast resistors can cause an increase in the xe2x80x9cknee voltagexe2x80x9d, which limits RF voltage swing amplitude and therefore PAE. Thermal shunt technique does not have the disadvantages associated with ballast resistors, and have demonstrated very high power density operation at 10 GHz with good PAE. However, thermal shunt HBTs have only marginal robustness under strong RF drive conditions.
In the prior art or conventional device, FIG. 1, the base current component of each subcell or transistor is a function of the local temperature. The local temperature, which is influenced by the power consumed in each subcell, is proportional to the collector current component. Because the temperature dependent current regulator (e-b junction) and the temperature generator (b-c junction) are in the same physical location, a strong positive electrothermal feedback exists.
The cascode operation of HBTs itself is not a new approach. Previously cascode HBT amplifiers were designed where common-emitter (CE) unit cells drive a common-base (CB) unit-cells of identical sizes. In this ordinary use of the cascode configuration multiple emitter CE cells provide the current for a similar sized multiple emitter CB cells. Therefore, the thermal instability is not eliminated. The present invention eliminates the thermal instability of HBTs by the use of a conceptually new cascode design.
The object of this invention is to produce a heterojunction bipolar transistor (HBT) having a high power gain and efficiency at microwave and millimeter wave frequencies while maintaining unconditional thermal stability, and provide robustness to electrical overstress (EOS).
These and other objectives are attained in the thermally stabilized cascode heterojunction bipolar transistor (TSC-HBT) by placing the current and power generation regions into separate temperature zones, thereby achieving thermal stability.